High-Energy Astrophysics at UBC
Demet successful defended her thesis on New tricks for old stars : studying compact objects through novel methodologies in timing, energy and imaging. She’s now off to work with the NuSTAR team at Caltech as a postdoctoral fellow. Congratulations Demet!
As reported in Astronomy and Astrophysics (2025A&A…693A.152G), analysis of IXPE observations and a comparison with visual measurements shows that the polarization of the X-ray emission from the Crab Pulsars traces the same path as the polarization in the visual as the star rotates. However, interestingly the X-ray polarization lags in its trip relative to the visual in the Crab interpulse, but not in the main pulse, revealing that the emission in these two phases originates in very different regions of the pulsar magnetosphere.
Leesa successfully defended her thesis on White dwarfs as probes of novel physical phenomena. She will be continuing her research at UBC. Congratulations Leesa!
As reported in Nature Astronomy (2024NatAs…8.1047H), IXPE observations reveal that the accreting X-ray pulsar is wobbling with a 35-day period and also precessing on a timescale of years. The fast wobble is similar to how a football wobbles as it flies through the air (so-called free precession) and the slower precession is similar to a top as the torque of the Earth’s gravity forces it to turn. In the case of Hercules X-1, the wobble reveals that the crust is asymmetric to a few parts in ten million, and the torques come from the material that the star is accreting.
We recently discovered an ultramassive white dwarf that escaped from the nearby Hyades cluster (2023ApJ…956L..41M). It is one of only three white dwarfs with a mass likely greater than 1.3 solar masses, but unlike the other two, it is not strongly magnetic, so we can use spectroscopy to estimate its mass as well, yielding a test of the mass-radius relation for white dwarfs!
Paul successfully defended his thesis on Studies of evolved stellar populations : from giants to remnants. He will be continuing his research at UBC. Congratulations Paul!
As reported in Nature (2023Natur.620…61C), the ZTF survey discovered a rapidly rotating white dwarf (ZTF J203349.8+322901.1 or Janus) that follow-up spectroscopy reveal is half-covered with helium and the other half-covered with hydrogen, a white dwarf with two faces!
IXPE finally measures the long-predicted polarization of X-rays from the surface and magnetosphere of a magnetar (4U 0142+61) as published in Science, 2022Sci…378..646T. The extent of polarization from 2-4 keV is about ten percent indicating the surface is likely to be solid or liquid. Above 4 keV, the mode of polarization switches and increases to about 30% as expected from resonant Compton scattering in the magnetosphere. This is an amazing validation and probe of the magnetar model!
Javiera defended her thesis on Red giant stars as standard candles. Congratulations Javiera!
We have completed a comprehensive study of massive white dwarfs with Gaia EDR3 10.1093/mnras/stac458. We can account for the numbers of white dwarfs between 0.95 and 1.15 solar masses with white dwarf cooling models and the rate of star formation in the Galaxy over the past few billion years. In fact, these massive white dwarfs provide an excellent method to trace star formation. The most massive and rare white dwarfs with masses greater than 1.15 solar masses appear to be form through a combination of mergers and the evolution of single stars with about equal numbers forming through these two avenues over the past few billion years.